In conventional examples, to form an elongated hole in a pipe as a workpiece (can manufacturing), a hole 1 is formed in the pipe so as to have such a shape as shown in a plane view (a lower view) of FIG. 1(a), and another material bent in the same shape is welded to manufacture a can.
However, according to the conventional can manufacturing method, since a box portion, which turns to a frame of an opening after the pipe is subjected to fusion cutting and an elongated hole opening portion is provided, is formed by welding, material degradation occurs due to the welding, or a highly skilled welding operator is necessary and forming requires a long time. Further, there also arises a problem that material loss occurs in the opening portion with a large opening actual dimension.
Therefore, applying a molding method adopted in conventional pipe arrangement and performing pulling machining has been considered. Since the molding adopted in the conventional pipe arrangement is circular forming and a stress of the pulling machining is provided at one central point only, it is sufficient to insert an arm from the center, connect it to a cored bar, and give pulling power. However, in molding of the elongated hole, balance cannot be kept in the pulling that uses force at one point, and the only method is gradually effecting the molding while performing adjustment (correction). Therefore, although a method for increasing the number of molding cores to two can be considered because the elongated hole is to be formed, if a connection method produces an error in length, distortion is accumulated in the arm that requires large force, a problem in proof stress occurs, a stress in molding that is applied to a machine further spreads to a workpiece, and normal molding is hard to be carried out. Therefore, a molding shaft has to be linear. Actually, an eccentric error applied to the workpiece in the early stage expands as the molding advances, and the connected shaft skids with larger force and bends after all.